The majority of cameras today use a single photodiode cell to perform light metering. The single photocell is arranged to measure the average of the lower center portion of the scene as illustrated in FIG. 1. This works reasonably well because consumers usually frame their subject in this part of the picture. One of the biggest problems with in-camera light meters is that they measure subject brightness rather than incident illumination. The light meters must be calibrated so that they correlate to the incident illumination. They are typically calibrated on the assumption that the scene reflectivity is 14%, which is not always a valid assumption. By averaging over smaller areas, the multi-cell photometer acquires more information about the scene contrast. This data can be used to categorize scenes and to apply the proper reflection factor.
Camera manufacturers have begun to utilize multiple photocells to measure segments of the scene independently. This is described in a number of journal articles, such as in the Camerart article, "How Does `Multi-Segment` Metering of SLRs Work", Feb. 1989 pp. 13-17. The patterns that have been employed leave room for improvement. Some of the patterns are designed to improve backlit subject detection, such as the Ricoh Mirai and the Minolta Dynax patterns illustrated in FIGS. 2 and 3, respectively. The pattern in the Canon EOS cameras, illustrated in FIG. 4, have the scene split into quadrants, which is useful for evaluating the subject against its foreground, the sky or background. Yet in all of these cases, if the subject is small or at a distance so that it appears small or further if the subject is not centered, or if the subject is two persons these patterns will not detect problematic lighting.
In Kawamura et al. U.S. Pat. No. 4,322,142, entitled "Single-Lens Reflex Camera Light Measurement and Exposure Control Apparatus" there is disclosed the use of three light measuring elements. The first element is used for the average measurement of the light reflected from a movable mirror during object observation. The second element is used for spot metering on an object from the light projected through the plane of the movable mirror, and the third element is located to receive reflected object light from the film surface and/or the shutter curtain surface. The patterns for the light measuring elements are shown in FIGS. 4A and 4B of the patent.
Another patent of interest is Tsunekawa U.S. Pat. No. 4,704,024, entitled "Photometric Circuit for Camera". In that patent there is shown a light meter that has a photo-electric conversion element for measuring the light in the middle part of the angle of view of the camera and a plurality of photo-electric conversion elements arranged to measure light in the peripheral parts of the angle of view. The patterns for these elements are shown in FIGS. 7A, 7B, 9, and 11 of the patent.
Also of interest are the teachings of Yamada et al. U.S. Pat. No. 4,774,401, entitled "Light Measuring Device Variably Controlled According to Focus Mode Selection". In that patent the light sensor uses silicon photodiodes that are divided into a central portion of the picture plane and three concentric parts around the central portion. FIGS. 1, 3, 5, and 6 of that patent illustrate the patterns used for the photoelements.